1. Field of the Invention
This invention relates to control structures for nuclear reactors and more particularly provides an inherent shutdown system responsive to off-normal operational core characteristics.
2. Description of the Prior Art
The fissioning reaction within a nuclear core can be controlled by selective addition and removal of neutron absorbing material, typically in the form of absorber rods. Rapid shutdown of the reactor during off-normal operating conditions is important, and recent systems have been proposed which inherently respond to such conditions to drop or otherwise insert absorber rods into the core. Typical of such systems is that described in U.S. Pat. No. 3,976,540 which discloses an inherent shutdown system including a neutron absorbing mass held above a core through a magnetic flux. The magnetic flux circuit includes a ferromagnetic Curie point material which inherently responds to a selected high temperature so as to lower its magnetic permeability and break the magnetic circuit, thereby allowing the magnet and a fixed absorber mass to fall, under the force of gravity and a spring assist, into the core region. Curie temperature materials are well known which can be selected or alloyed to have a Curie point temperature.
While this and similar teachings appear to provide desired inherent shutdown capabilities, improvements can be made. For example, the discussed patent mixes coolant passing over fuel rods with cooling flowing through a vacant central region into which the absorber mass will drop. In the event of overtemperature conditions in the fuel rods, the coolant flowing about the fuel rods will heat up but be diluted by the cooler coolant flowing through the vacant region, thus slowing down the response time to reach the selected Curie temperature. Further, the teaching proposes lateral support of the magnet assembly and a fixed absorber mass which is dependent upon frictional characteristics and may thus require a larger holding force than otherwise necessary. The system additionally requires springs to laterally displace a portion of the holding structure to allow free fall of the absorber mass. The system also drops the magnet as well as the absorber mass into the core, exposing the magnet to a high and potentially damaging neutron flux upon insertion. Also, a mechanical manipulator is required to retrieve the magnet and absorber mass subsequent to insertion. And, relatively large surface area contact exists among the absorber mass and magnet and the structures which retain them in an upper position, potentially resulting in self-welding during normal operation which would defeat the insertion. Additionally, to test the system the Curie temperature material must be resistance heated. Alternatively, although not taught, if the magnet is an electromagnet allowing testing by cutting of the electrical power, the power leads will drop along the magnet, creating high mechanical loadings and exposing the leads to a high neutron flux and temperature upon insertion into the core.
It is thus desirable to provide an improved inherent shutdown system which alleviates these, and other deficiencies, in the present nuclear reactor inherent shutdown systems.